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Abstract

The effects of high hydrostatic pressure on excitatory neuromuscular
transmission in shallow- and deep-living crustaceans were compared.
Pressure caused depression of the amplitude of excitatory junctional
potentials (e.j.p.s) at the neuromuscular junction in the shallow-living
crab, Libinia emarginata. A pressure of 100 atm depressed the e.j.p.
amplitude by about one-half.
In the deep- sea crab, Geryon quinquedens, which ranges to a depth of
2000 m (or 200 atm pressure), adaptations to high pressure were observed in
two different types of muscle fibers: 1) In fibers with e.j.p.s that showed
high levels of facilitation, the magnitude of pressure-induced depression decreased
with increasing frequency of nerve stimulation; i. e., there was a
pressure-induced increase in facilitation. Also a pressure-induced increase
in the duration of the falling phase of the e.j.p. was observed which served
to increase the level of depolarization resulting from summation of the
e.j.p.s at high frequencies of nerve stimulation. In these highly facilitating
fibers the physiologically significant frequencies that cause appreciable
contraction are probably high. At high frequencies the pressure-induced
increases in facilitation and summation together served to completely
counteract the depressive effect of pressure, and the net depolarization
attained during a train of nerve stimulation was relatively unaffected by
pressures up to at least 200 atm.
2) Fibers with e.j.p.s showing low levels of facilitation may undergo
significant contraction at low frequencies of nerve impulses where neither
facilitation nor summation play a significant role. The amplitude of
e.j.p.s recorded from this fiber-type in the deep-sea crab were, on the
average, unaffected by pressures to 200 atm. The e.j.p.s of some of these
fibers showed depression, but others were amplified under pressure.
The results of experiments with the lobster, Homarus americanus,
which ranges to a depth intermediate between Libinia and Geryon were in
many respects intermediate between the results obtained with the two
species of crab. Studies of the effect of pressure on isometric tension
developed by whole muscles in Homarus and Geryon were consistent with
the results of the studies of the e.j.p.; pressure depressed the rate of
rise of tension in Homarus and had little effect in Geryon.
The results of this work provides a physiological basis for the observation
that shallow-living animals are generally immobilized by pressures
in excess of 200 atm.
Experiments were performed in an attempt to elucidate the mechanism
underlying the pressure-induced depression of e.j.p. amplitude. Results
were suggestive that the depression of e.j.p. amplitude reflects a pressure-induced
decrease in the number of quanta of transmitter substance released
by the nerve endings.

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Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution November, 1975

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